This invention relates to a proportional modulator for controlling air pressure in a braking system and particularly to a modulator of simplified design having direct exhaust capability.
Electropneumatic braking systems (EBS) use a fluid modulator that converts an electrical signal from an electronic control unit into a pneumatic signal that actuates vehicle brakes. A conventional electropneumatic proportional brake for regulating pressure is shown and described in U.S. Pat. No. 5,123,718, the details of which are incorporated herein by reference. An electrical signal is conveyed from a foot pedal and input to an electronic control unit which provides an electrical signal to an armature of a solenoid actuator of the modulator. Energizing the armature controls movement of a piston extending outwardly from the solenoid valve assembly. The piston includes a seal that engages a valve seat formed on a hollow spool valve. Opposite external end surfaces of the spool valve include seal members, such as O-rings, that are sealed relative to the modulator housing in a deactuated position of the spool valve. Thus in the deactuated position, supply pressure from a source of pressurized air is communicated to an inlet port of the modulator and does not reach the brake or delivery port. Instead, the delivery port remains in communication with an exhaust port, i.e., the brakes are in a release position.
A diaphragm includes a pilot passage therethrough which maintains the delivery port in communication with a small diameter exhaust port formed in the modulator housing. More particularly, when deactuated the piston is retracted relative to the solenoid valve. Thus, the piston is disposed in a normally open position. In this manner, the delivery port is in communication through a small diameter passage formed in the hollow spool valve with the small diameter exhaust port.
During a brake application, a pulse width modulated electrical signal is provided to the solenoid armature. This urges the piston to a closed position with the valve seat on the end of the spool valve. The piston also moves the spool valve so that the external seal on the spool valve is opened and supply pressure communicates therethrough to the delivery port. As will be appreciated, the diaphragm is also unseated as a result of the pressure from the supply port. The pressure from the supply port also communicates through a small diameter opening through the spool valve. The pressure, in turn, acts against the piston so that a proportioning action occurs.
When the brake is released and the piston is retracted within the armature, the spool valve engages its seat and pneumatic pressure through the supply port is terminated. The pressure at the delivery port lifts the diaphragm valve so that a quick exhaust may be made through the large diameter exhaust opening in the modulator housing.
The conventional EBS proportional modulator assembly described above is relatively complex and has many components. The diaphragm used as a quick release also adds an undesired differential across the supply and delivery ports. In addition, it is desirable to eliminate the small diameter exhaust port opening through which the brake port normally communicates through the spool valve.
Accordingly, a need exists for a quick release valve of an ABS/EBS module that exhausts a large control volume in a short period of time without adding a differential across the supply and delivery ports. It is desirable that such a valve not be as complex as that known in the art.